In particular the cloud-free inner boundary of the HZ is moved outward as compared to the Kasting et al. (1993) estimates. The onset of "moist greenhouse" conditions is predicted at 0.99 AU (versus 0.95 AU) and the runaway greenhouse at 0.97 AU (as opposed to 0.84 AU). However:

Although it appears that Earth is perilously close to the inner HZ edge, in reality, cloud feedback and low upper tropospheric relative humidity act to stabilize Earths climate.

For stellar temperatures below 5000 K, the distinction between the two inner boundaries disappears.

According to figure 8, Earth, HD 40307g, Gl 581d and Gl 581g are in the HZ for cloud-free planets. Kepler-22b, Gl 667 Cc, and Tau Ceti e and f are not. As for the case of clouds:

Our model does not include the radiative effects of clouds; thus, the actual HZ boundaries may extend further in both directions than the estimates just given.

I am sceptic about any estimates of hz of any star.In case of our solar system hz can be wide enough to contain 3 planets.90 bars pressure of Venusian atmosphere can not be any proof in which life can not have comfortable condition to develop at 0.7-0.8 AU if day would be normal(not243 days) and composition of air would be different.Example of Venus and mars are freak.On the other side mars has very thin atmosphere 10 times more pressure and would change all.The cases of Venus and mars will not happen in any other planetary systems as there could be more nitrogen ,oxygen and water

Last edited by tommi59 on 29th January 2013, 11:58 am; edited 1 time in total

Well given the increase in solar luminosity over the last 4.5 billion years or so it maybe shouldn't be too surprising that the Earth is close to the inner edge of the HZ. No particular reason we should be in the optimum part of the HZ (and I really have to wonder what life would be like on the presumably more CO2-rich worlds further out in the HZ, I'd guess for starters that ocean acidity might have an effect on the evolution of things like shells).

Of course if you end up showing Earth is not a habitable planet you've really demonstrated that there's a flaw in your model (which the authors of the paper themselves specifically take pains to point out, i.e. the lack of cloud modelling). Nevertheless, even without the clouds Earth does end up being just inside the habitable zone despite what hyperbolic headline writers are saying...

They(scientists)mostly if not all assess low albedo considering habitability up to 0.3.Planets with radii between 1.3-3.0 surely are water worlds with much thicker clouds content and some even ice planets they have higher albedo even as high as saturns moons

You could also in principle get a higher albedo from Rayleigh scattering in a sufficiently thick atmosphere. E.g. this paper (pdf) gives a cloud-free albedo for 0.4 (higher than the Earth's albedo) mainly as a result of Rayleigh scattering.

Like Lazarus wrote this is the case I guess most planets if not all within range 1.3-3.0 earth radius have surely much thicker atmosphere than earth especially in hz where is no mass loss zone due all stellar irradiation.So seems to be that planets with radii over 1.5 have higher albedo and under 1.2 lower.I think earth has thin atmosphere and most planet with even similar radius can have higher albedo